Control method and device for switching power supplies

ABSTRACT

A control device for controlling a switching power supply adapted to convert an input voltage into an output voltage according to a switching rate of a switching element. The control device includes first control means for switching the switching element in a first working mode at a constant frequency and second control means for switching the switching element in a second working mode at a variable frequency, under a maximum frequency, in response to the detection of a predefined operative condition of the switching power supply. The control device further includes means for selecting the first working mode or the second working mode.

TECHNICAL FIELD

Embodiments of the present invention relate generally to the field ofelectronics, and more specifically to the control of switching powersupplies.

BACKGROUND

Switching power supplies are widely used to provide regulated voltagesand currents to circuit loads. A switching power supply converts aninput voltage by temporarily storing energy corresponding to the inputvoltage and then releasing that energy to the load, so as to obtain a DCoutput voltage. The storage of energy (carried out with either magneticcomponents, like inductors and transformers, or capacitors) iscontrolled by means of a switching element (such as a power MOSFET).

Different circuit topologies are known within the class of the switchingpower supplies. A well-known topology is that referred to as “flybacktopology”. As known to those skilled in the art, a switching powersupply having the flyback topology, also referred to as “flybackconverter”, may be directly connected to a main (AC) voltage source, andits (DC) input voltage is obtained by rectifying and filtering the mainvoltage by means of an input network including a rectifier bridge and acapacitive filter. A flyback converter includes an electric transformer,with the switching element that periodically connects a primary windingof said transformer to the input network (providing the input voltage),in such a way to modulate the energy that is transferred to a secondarywinding coupled to the load.

Generally, the flyback converter, like all the switching power supplies,requires a control circuit that carries out the task of properly drivingthe switching element.

The simplest control circuits of such type—herein referred to as“constant frequency (CF) control circuits”—are designed for driving theswitching element with a constant frequency. In this case, the controlcircuit has a simple circuit architecture, including for example anoscillator and a flip-flop that controls a driver connected to theswitching element. Thus, the switching element is opened and closedaccording to the frequency of the oscillator; by regulating thefrequency of the oscillator, it is possible to regulate the transfer ofenergy, and, thus, the level of the output voltage. The CF controlcircuit has the advantage of allowing the flyback converter to workaccording to two different modes.

Particularly, the flyback converter may work in the so-called ContinuousConduction Mode (CCM) or in the so-called Discontinuous Conduction Mode(DCM), depending on the behavior of a current flowing through thesecondary winding during the switching cycle of the switching element:in the CCM, the current flows through the secondary winding for thewhole period occurring between the opening of the switching element andthe closure thereof. Conversely, in the DCM, the current in thesecondary winding diminishes so as to reach a null value before theending of said period.

The DCM is usually exploited when the powers to be managed are low,since in this case it is possible to reduce the sizes of the magneticcomponents of the converter. For higher powers the CCM is insteadpreferable, since it allows obtaining a more advantageous current'sshape factor.

A more complex family of control circuits for flyback convertersconsists of control circuits—herein referred to as “Quasi-Resonant (QR)control circuits”—designed for driving the switching element with afrequency that varies over time, depending on the magnetization statusof the transformer (which in turn depends on the input voltage and theoutput current). Particularly, the QR control circuit synchronizes, witha proper delay, the switching of the switching element with the instantat which the current flowing in the secondary winding reaches a nullvalue, in such a way that the switching element is activated when avoltage at a terminal of the switching element connected to the primarywinding is near zero. The latter condition is also known as Zero VoltageSwitching (ZVS) condition.

Examining in greater detail the behavior of the switching element duringthe operation of the flyback converter, the voltage at the terminal ofthe switching element connected to the primary winding—herein referredto as “switching voltage”—starts to oscillate with damped oscillationsimmediately after the instant at which the current flowing in thesecondary winding reaches the null value. In this way, the switchingvoltage exhibits a sequence of falling edges followed by correspondingrising edges, which determine a corresponding sequence of minimumvalues, each one representing a possible ZVS conditions.

Since the switching element driven by the QR control circuit isactivated very near the instant at which the current of the secondarywinding has reached the null value, the operation of the flybackconverter corresponds to the boundary between the CCM and DCM. Usually,a flyback converter driven by the QR control circuit has to work withpowers under 100 Watts.

Compared to the CF control circuit, the QR control circuit has severaladvantages.

The most important advantage regards the electromagnetic compatibility.Indeed, being the working frequency variable depending on the inputvoltage and the output current, the energy irradiated by the converterbelongs to a plurality of energy bands. In this way, instead of beingconcentrated at a single frequency with a high intensity, theelectromagnetic emission is distributed among different energy bandswith much lower intensities.

Another significant advantage given by the utilization of the QR controlcircuit regards the possibility of switching the switching element inthe ZVS condition. Indeed, since the switching element is usually aMOSFET connected between the primary winding and a ground terminal, inthe ZVS condition the voltage across the MOSFET is near to zero, andthus the capacitive losses due to its drain-to-source parasiticcapacitance are negligible.

In the variable-frequency operation of the flyback converter, theswitching frequency increases when the output load is low ant the inputvoltage is high. This increase can drastically increment the losses dueto parasitic elements of the flyback converter.

This situation is usually addressed by introducing a circuit(implementing a so-called valley skipping technique) that sets a minimumperiod during which the switching element has to remain activated. Inthis way, if the ZVS condition occurs before the end of the minimumperiod, the ZVS condition is ignored, and the switching element remainsdeactivated. More particularly, the switching element is activated atthe occurrence of the ZVS condition corresponding to the first minimumvalue—in the sequence determined by the oscillations of the switchingvoltage—after the expiration of the minimum period.

At the present time, the choice of which type of control circuit to beused—between the CF type and the QR type—depends on several anddifferent factors, like for example the personal skill of a technician,the complexity and the cost of the circuit and the amount of power thathas to be transferred.

More complex structures of the control circuit have also been proposed.

An example of a control circuit for flyback converters known in the artis the device TEA1654 by Philips. This device is a QR control circuitthat normally operates at a variable switching frequency. However, ifthe switching frequency exceeds a predetermined value, the switchingfrequency is locked to the value and the device operates in the CF mode.This solution has the great drawback that in the latter condition theswitching element may be activated when the switching voltage has avalue that is significantly higher than zero (with significantcapacitive losses).

Another control circuit for flyback converters known in the art is thedevice TDA16846/TDA16847 by Infineon. This device is again a QR controlcircuit that operates at a variable switching frequency. Moreover, thedevice is capable of always operating in the QR mode, but in such a wayto control the switching element with a switching frequency that is asclose as possible to a predetermined value. However, since the devicealways drives the switching element following the occurrences of the ZVScondition, it is not possible to use it for controlling the flybackconverter in the CCM.

SUMMARY

More specifically, an embodiment of the present invention provides acontrol device for controlling a switching power supply. The switchingpower supply is adapted to convert an input voltage into an outputvoltage according to a switching rate of a switching element. Thecontrol device includes first control means for switching the switchingelement in a first working mode at a constant frequency, and secondcontrol means for switching the switching element in a second workingmode at a variable frequency, under a maximum frequency, in response tothe detection of a predefined operative condition of the switching powersupply. The control device further includes means for selecting thefirst working mode or the second working mode.

In an embodiment of the invention, the constant frequency and themaximum frequency depend on a timing frequency of a common timingsignal.

The switching power supply may have a flyback architecture including atransformer, and the control device may include means for detecting azero current condition in a secondary winding of the transformer.

According to an embodiment of the present invention, means is providedfor detecting stationary points in an auxiliary voltage of an auxiliarywinding (oscillating in response to the zero current condition).

In an embodiment of the invention, the first control means comprisesmeans for switching the switching element in response to the timingsignal, and the second control means comprises means for switching theswitching element in response to a first stationary point in theauxiliary voltage after a minimum period corresponding to the maximumfrequency.

According to an embodiment of the invention, an oscillator is providedfor generating the timing signal. Furthermore, the second control meansincludes means for resetting said oscillator in response to theswitching of the switching element.

The switching element may be switched by the second control means aftera maximum period (that depends on the timing frequency) without thedetection of any stationary point.

Means for switching the switching element in response to a power-on ofthe power supply may be included in the second control means.

According to an embodiment of the present invention, the control devicecomprises a first input terminal for receiving a frequency indicationfor setting the timing frequency. In an embodiment of the invention, asecond input terminal for receiving a working mode indication of thecontrol device is provided. The second input terminal may be furtheradapted to receive the auxiliary voltage. Another embodiment of thepresent invention provides a power conversion system including thecontrol device and a switching power supply. A further embodiment of thepresent invention provides an electronic apparatus including the system.A still further embodiment of the present invention provides acorresponding method for controlling a switching power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be madeapparent by the following detailed description of an embodiment thereof,provided merely by way of non-limitative example, which will beconducted making reference to the annexed drawings, wherein:

FIG. 1 schematically shows the main circuit components of a flybackconverter that can be driven by a control circuit according to anembodiment of the present invention;

FIG. 2 is a timing diagram illustrating how the flyback converter ofFIG. 1 is driven when the control circuit works in the CF mode;

FIGS. 3A and 3B are timing diagrams illustrating how the flybackconverter of FIG. 1 is driven when the control circuit works in the QRmode; and

FIG. 4 is a schematic diagram of the main functional blocks included inthe control circuit of FIG. 1 according to an embodiment of the presentinvention.

DETAILED DESCRIPTION

The following discussion is presented to enable a person skilled in theart to make and use the invention. Various modifications to theembodiments will be readily apparent to those skilled in the art, andthe generic principles herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentinvention. Thus, the present invention is not intended to be limited tothe embodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein.

With reference to the drawings, FIG. 1 schematically shows the maincircuit components of a flyback converter 100 that can be driven by acontrol circuit 105 (in the form of a corresponding device, for exampleintegrated in a chip of semiconductor material) according to anembodiment of the present invention.

For the sake of convenience, in the following of the description thevoltages and the circuit nodes providing them will be denoted with thesame references.

The flyback converter 100 includes an input rectifier block 110, whichreceives an AC voltage Vac from an AC main source (not shown in thefigure) and provides a corresponding DC input voltage Vin. Moreparticularly, the voltage Vac is fed to an input of a rectifier circuit115—for example, a diode bridge—whose output is coupled to a filtercircuit 120—for example, a capacitive filter. The rectifier circuit 115rectifies the voltage Vac into an output voltage with a fixed polaritybut with a pulsating magnitude. The voltage is then filtered by means ofthe filter circuit 120, so as to smooth the variation in the voltageprovided by the rectifier circuit 115 and obtain the input voltage Vinof which the amplitude is substantially constant.

The flyback converter 100 further includes a transformer 125 having thepurpose of transforming the input voltage Vin into a correspondingoutput voltage Vout with a different (particularly, lower) value. Thetransformer 125 comprises a primary winding 130 magnetically coupledwith a secondary winding 135. The primary winding 130 has a firstterminal connected to the rectifier block 110 for receiving the inputvoltage Vin, and a second terminal that is brought to a switchingvoltage Vsw. The secondary winding 135 has instead the terminalsconnected to an output stage 140 for providing the output voltage Voutthereto.

The output stage 140 has the purpose of generating a DC regulated outputvoltage Vrout from the output voltage Vout. The output stage 140includes a diode 145 having an anode terminal connected to a firstterminal of the secondary winding 135 and a cathode terminal connectedto a first terminal of an output capacitor 147. The second terminals ofthe secondary winding 135 and of the output capacitor 147 are connectedtogether to a reference (or ground) terminal. The output capacitor 147provides the regulated output signal Vrout to an output load (not shownin the figure).

The flyback converter 100 further includes a switching element 150, forexample, a power MOSFET. The MOSFET 150 has a drain terminal connectedto the second terminal of the primary winding 130 for receiving theswitching voltage Vsw, and a source terminal connected to a firstterminal of a feedback resistor Rf and to an input terminal of thecontrol circuit 105 for providing an open voltage Vop, whose purposewill be described in the following of the description. The MOSFET 150further includes a gate terminal that receives (from an output terminalof the control circuit 105) a driving signal Vdr whose value determinesthe opening or the closure of the MOSFET 150. The feedback resistor Rfhas a second terminal connected to the ground terminal.

When the MOSFET 150 is closed, i.e., when it is activated and the drainterminal is electrically coupled with the source terminal, the primarywinding 130 receives (from the input rectifier block 110) a current thatgenerates a corresponding magnetic flux increasing the energy stored inthe transformer 125. During this operating phase, and according to thereciprocal polarities of the windings 130 and 135 that are shown in thefigure, the voltage applied to the diode 145 reverse biases it.Moreover, during this phase, the output capacitor 147, which storeselectric charge received in a previous phase, discharges onto the loadso as to provide the requested current.

When the MOSFET 150 is opened, i.e., when it is deactivated, and thedrain terminal is electrically insulated from the source terminal, thecurrent flowing into the primary winding 130 goes to zero. Moreover, theenergy stored in the transformer 125 is transferred to the output load,which receives a current flowing from the secondary winding 135 throughthe diode 145 (being now forward biased). At the same time, thecapacitor 147 is charged by the current coming from the secondarywinding 135.

Consequently, the regulated output voltage Vrout takes a value thatstrongly depends on the duty cycle of the driving signal Vdr.

The transformer 125 further includes an auxiliary winding 155 that ismagnetically coupled with the primary winding 135 in such a way to betraversed by the same magnetic flux flowing therethrough. The voltagebetween the terminals of the auxiliary winding is provided to a voltagedivider circuit 160 (for example, a resistive divider) to obtain acorresponding auxiliary voltage Vaux. According to the reciprocalpolarities of the windings 130 and 155 that are shown in the figure, andaccording to the voltage ratio determined by the divider circuit 160,the amplitude of the auxiliary voltage Vaux is proportional to thevoltage between the first and second terminals of the primary winding(equal to Vin−Vsw) and has an opposite polarity. The auxiliary voltageVaux is applied to another input terminal of the control circuit 105.

The control circuit 105 further includes an input terminal for receivinga setting signal SET whose purpose will be apparent in the following.

According to an embodiment of the present invention, the control circuit105 is capable of driving the flyback converter 100 according to twodifferent working modes (being selectable by a user of the flybackconverter 100).

More particularly, as will be described in greater detail in thefollowing description, according to a first working mode—referred to as“constant frequency (CF) mode”, in relationship with the operation ofthe CF control circuits described in the introduction of thisdocument—the MOSFET 150 is driven by the control circuit 105 in such away to switch between the open and the closed conditions with a constantswitching frequency. Moreover, according to a second workingmode—referred to as “Quasi-Resonant (QR) mode”, in relationship with theoperation of the QR control circuits previously described—the MOSFET 150is driven by the control circuit 105 with a switching frequency thatvaries over time, depending on the magnetization status of thetransformer 125.

In this way, the same flyback converter 100 can be used in differentscenarios. Indeed, the user of the flyback converter 100 is able to setthe preferred working mode, based on the characteristics of the load andthe voltage to be converted.

In this way, the flyback converter 100 can provide the best performancein every scenario. For example, it is possible to work normally in theQR mode (between the CCM and the DCM). Moreover, it is possible toswitch to the CF mode, so as to work in the CCM for high powers.

Reference will be now made to FIG. 1 together with FIGS. 2, 3A and 3B,which are timing diagrams illustrating how the flyback converter 100 isdriven when the control circuit 105 works in the CF and QR modes.

As will be described in greater detail in the following description, allthe operations carried out by the control circuit 105—both in the CF andin the QR modes—are based on a clock signal OSC, which is internallygenerated in the control circuit 105 itself.

FIG. 2 shows how the control circuit 105 operates in the CF mode.

In the CF mode, the clock signal OSC oscillates with a period Tcf, whichdirectly determines the switching frequency at which the MOSFET 150 ofthe flyback converter 100 switches between the open and the closedconditions. More particularly, according to this mode, the drivingsignal Vdr is generated based on the rising edges of the clock signalOSC: every time the clock signal OSC exhibits a rising edge, the drivingsignal Vdr is brought to a high value, in such a way to close or turn ONthe MOSFET 150. The driving signal Vdr remains at the high value for aperiod Tclosed that depends on the current flowing through the primarywinding 130 of the transformer 125. In particular, as far as the MOSFET150 is in the closed condition, the current flowing through the primarywinding 130 of the transformer 125 increases, so that the open voltageVop accordingly increases. When the open voltage Vop reaches apredetermined value, the driving signal Vdr is brought to a low value,in such a way to open or turn OFF the MOSFET 150. The MOSFET 150 thenremains open (for a period Topen) until the next rising edge of theclock signal OSC. Consequently, by varying the frequency of the clocksignal OSC it is possible to vary the switching frequency at which theMOSFET 150 switches, and, consequently, the transfer of electric powerfrom the primary winding 130 to the secondary winding 135.

FIGS. 3A and 3B show two different examples of how the control circuit105 operates in the QR mode.

Referring in particular to FIG. 3A, the clock signal OSC oscillatesbetween a high value and a low value with a variable duty cycle, whichdepends on the switching voltage Vsw. More particularly, the period atwhich the clock signal OSC is at the high value—referred to as highperiod Th—is kept constant, and can be set to a desired value by theuser of the flyback converter 100. On the contrary, the period at whichthe clock signal OSC is at the low value—referred to as low periodTl—varies, since it is determined by the occurrence of a ZVS condition.

More specifically, when the MOSFET 150 is in the closed condition, i.e.,when the driving signal Vdr is at the high value, the switching voltageVsw is low, being substantially equal to the ground voltage. As in theCF mode, the driving signal Vdr remains at the high value for a periodTclosed that depends on the open voltage Vop, and then on the currentflowing through the primary winding 130 of the transformer 125. When thedriving signal Vdr reaches the low value, the MOSFET 150 switches to theopen condition. In this condition, the switching voltage Vsw rises,reaching—after a transient period characterized by the presence ofoscillations—a stable value that depends on the input voltage Vin.During this period, the energy stored in the transformer 125 istransferred to the load, which is fed by a current provided by thesecondary winding 135. As known to those skilled in the art, when thecurrent flowing in the secondary winding 135 reaches the null value—thiscondition is referred to as “Zero Current” (ZC) condition—, theswitching voltage Vsw starts to oscillate with damped oscillations.Consequently, the switching voltage Vsw exhibits a sequence of fallingedges followed by corresponding rising edges—which determine acorresponding sequence of minimum values. Since each minimum value ofthe sequence corresponds to a substantially null voltage, acorresponding sequence of possible ZVS conditions is established.

As will be described in the following description, the control circuit105 is able to identify the ZVS conditions by analyzing the behavior ofthe auxiliary voltage Vaux, since the amplitude of the auxiliary voltageVaux is proportional to Vin−Vsw. At the instant at which the switchingvoltage Vsw has the first minimum value in the sequence, the controlcircuit 105 checks if the clock signal OSC is at the low value or not.

In case the clock signal OSC is at the low value—that is, in case thehigh period Th is expired—the driving signal Vdr is driven to the highvalue, in such a way that the MOSFET 150 is switched to the closedcondition.

Conversely, in case the clock signal OSC is at the high value—i.e., incase the minimum value occurs when the high period Th is not expiredyet—the driving signal Vdr is kept at the low value, so that the MOSFET150 remains in the open condition. In this case, the control circuit 105repeats the same operations for the next minimum value of the switchingvoltage Vsw in the sequence.

In the example illustrated in the FIG. 3A, the first minimum value ofthe switching voltage Vsw in the sequence occurs when the clock signalOSC is at the low value, so that the driving signal Vdr is switched tothe high value immediately. Conversely, in the example illustrated inthe FIG. 3B both the first two minimum values of the switching voltageVsw in the sequence occur when the clock signal OSC is at the highvalue. In this case, the driving signal Vdr is switched to the highvalue at the third minimum value, which occurs when the clock signal OSCis at the low value.

When the driving signal Vdr is driven to the high value, the clocksignal OSC is reset to the high value. In this way, the clock signal OSCis always synchronized with the rising edges of the driving signal Vdr.

Using this solution (also referred to as “valley skipping”, since eachminimum value is a stationary point placed at the bottom of a “valley”),it is possible to establish the maximum switching frequency of theMOSFET 150 by setting the duration of the high period Th of the clocksignal OSC. It has to be appreciated that this solution allows theMOSFET 150 to switch always in a ZVS condition, so that the capacitivelosses are reduced.

Referring now to FIG. 4, a schematic diagram of the main functionalblocks included in the control circuit 105 is illustrated according toan embodiment of the present invention.

The control circuit 105 includes an oscillator block 405 coupled with aninternal clock generator block 407. The oscillator block 405 has thepurpose of generating the clock signal OSC based on the frequency of aninternal clock signal ICK generated by the clock generator block 407.According to an embodiment of the present invention, the frequency ofthe internal clock signal ICK, and thus the frequency of the clocksignal OSC, can be set by means of the setting signal SET, which is fedto the internal clock generator block 407 from the outside. For example,the frequency of the internal clock signal ICK may be proportional tothe amplitude of the setting signal SET. It has to be appreciated that,according to this solution, the operations carried out by the controlcircuit 105 in the CF mode and in the QR mode are based on a samesignal, i.e., the internal clock signal ICK. Indeed, the clock signalOSC, whose behavior varies according to the selected mode, alwaysdepends on the frequency of the internal clock signal ICK. In this waythe control circuit 105 has to include a single internal clock generatorblock 407.

The clock signal OSC is provided to a QR block 410 and to a first inputof a selector 415, for example a multiplexer.

The QR block 410 is in charge of the management of the operationsnecessary for driving the flyback converter in the QR mode. The QR block410 further receives the auxiliary voltage Vaux—from the voltage dividercircuit of the flyback circuit—and accordingly generates a QR drivingsignal QRDR that is provided to a second input terminal of the selector415.

Depending on a chosen mode between the CF and the QR modes, the clocksignal OSC or the QR driving signal QRDR are selected by the selector415 and are provided to a set input of a SR flip-flop 420. Particularly,according to the value of a selection signal SEL provided to a selectionterminal of the selector 415, the set input of the SR flip-flop 420 isfed with the clock signal OSC (CF mode) or with the QR driving signalQRDR (QR mode). For this purpose, the control circuit 105 includes abuffer block 422 that is adapted to receive the selection signal SELfrom the outside and to latch its value. According to an embodiment ofthe present invention, the selection signal SEL is fed to the bufferblock 422 from the outside through the same terminal adapted to receivethe auxiliary voltage Vaux. In this case, the values the selectionsignal SEL is expected to take must be different with respect to thoseof the voltage range defined by the auxiliary voltage Vaux.

The SR flip-flop 420 further includes a reset terminal receiving a resetsignal RES from an output of a comparator 423 and an output terminalcoupled to a driver 425. The driver 425, for example, a non-invertingbuffer circuit, is adapted to generate the driving voltage Vdr inresponse to the output of the SR flip-flop 420. The comparator 423 has afirst input for receiving (from the flyback converter) the open voltageVop, and a second input receiving a predetermined reference voltageVref. When the open voltage Vop is lower than the reference voltageVref, the reset signal RES is at the low value. Conversely, as soon asthe open voltage Vop becomes higher than the reference voltage Vref, thereset signal RES switches to the high value.

The QR block 410 includes a zero detector block 430 receiving theauxiliary voltage Vaux, and outputting a zero detection indication ZDeach time the auxiliary voltage Vaux exhibits a falling edge followed bya consecutive rising edge. Circuits of this type are known to thoseskilled in the art, and usually include a comparator with hysteresis.

The zero detection indication ZD is provided to a valley skip block 435and to an exceptions management block 440. Both the valley skip block435 and the exceptions management block 440 also receive the clocksignal OSC from the oscillator block 405. The exceptions managementblock 440 still further receives the internal clock signal ICK from theclock generator block 407.

Based on the value of the clock signal OSC, the valley skip block 435may ignore the zero detection indication ZD or not, as it has beendescribed in greater detail with reference to the FIGS. 3A and 3B. Ifthe zero detection indication ZD occurred when the clock signal OSC isat the low value (meaning that the corresponding ZVS condition respectsthe maximum frequency requirements), the valley skip block 435 providesa (positive) pulse to a first input of an OR logic port 445, whoseoutput generates the QR driving signal QRDR.

The exceptions management block 440 has the purpose of generating a(positive) pulse to a second input of the OR logic port 445 in case thezero detector block 430 does not generate a zero detection indication ZDafter a predetermined period—e.g., after a predetermined number ofperiods of the internal clock signal ICK—because the oscillations of theauxiliary voltage Vaux are too damped. Moreover, the exceptionsmanagement block 440 outputs a pulse to the second input of the OR logicport 445 at the power-on of the flyback converter.

The output of the OR logic port 445 is further connected to a resetterminal of the oscillator block 405, in such a way that when the QRdriving signal QRDR goes to the high value (i.e., when the valley skipblock 435 or the exceptions management block 440 output a positivepulse), the oscillator block 405 is reset, that is, the clock signal OSCis switched to the high value and a new high period Th is initiated.

When the CF mode is selected the set terminal of the SR flip-flop 420receives the clock signal OSC, and its reset terminal is fed with thereset signal RES. Thus, as it has been described with reference to FIG.2, the driving signal Vdr is made to switch to the high value every timethe clock signal OSC exhibits a rising edge, and to the low value everytime the open voltage Vop reaches the value of the reference voltageVref. In addition, the driving signal Vdr may be also made to switch tothe low value after a predetermined maximum period—for example,proportional to the period of the internal clock signal ICK—is expired.

When the QR mode is selected, the set terminal of the SR flip-flop 420is fed with the QR driving signal QRDR, and its reset terminal is fedagain with the reset signal RES. Thus, as it has been described withreference to FIGS. 3A and 3B, the driving signal Vdr is made to switchto the high value every time a ZVS condition has been established (i.e.,when a zero detection indication ZD is generated) under the maximumfrequency requirements established by the high period Th of the clocksignal OSC. In any case, the driving signal Vdr is made to switch to thehigh voltage after the maximum period without the detection of any ZVScondition. Moreover, the driving signal Vdr switches to the high valueimmediately after the power-on of the flyback converter (when no ZVSconditions are still established). As above, the driving signal Vdr ismade to switch to the low value every time the open voltage Vop reachesthe value of the reference voltage Vref.

The control circuit 105 herein proposed requires a reduced number ofinput/output terminals for receiving/providing voltages from/to theflyback converter. Particularly, the control circuit 105 includes aterminal for providing the driving signal Vdr, a terminal for receivingthe open voltage Vop, a terminal for receiving the auxiliary voltageVaux and a terminal for receiving the setting signal SET.

Naturally, in order to satisfy local and specific requirements, a personskilled in the art may apply to the solution described above manylogical and/or physical modifications and alterations. Morespecifically, although the present invention has been described with acertain degree of particularity with reference to preferredembodiment(s) thereof, it should be understood that various omissions,substitutions and changes in the form and details as well as otherembodiments are possible. Particularly, the proposed solution may evenbe practiced without the specific details (such as the numericalexamples) set forth in the preceding description to provide a morethorough understanding thereof. Conversely, well-known features may havebeen omitted or simplified in order not to obscure the description withunnecessary particulars. Moreover, it is expressly intended thatspecific elements and/or method steps described in connection with anydisclosed embodiment of the invention may be incorporated in any otherembodiment as a matter of general design choice.

Particularly, similar considerations apply if the control device has adifferent structure or includes equivalent elements. Moreover, the inputvoltage and/or the output voltage can take any value. In any case, theMOSFET may be replaced with a bipolar transistor, or more generally withany other equivalent switching element.

Even if reference has been made to a control device for a flybackconverter, similar considerations can be applied to different types ofswitching power supplies.

In a very simplified implementation, it is also possible to provide twodistinct oscillators for the CF mode and the QR mode.

The concepts of the present invention are also applicable in case thezero current condition is detected in another way, for example, bydirectly analyzing the input voltage of the primary winding.

In any case, concepts of the present invention are also applicable incase the means for detecting the stationary points (i.e., minimum ormaximum values) in the auxiliary voltage are capable of detecting astationary point different than the first stationary point in theauxiliary voltage after the minimum period defined by the maximumfrequency.

Similar considerations apply if the oscillator is replaced with anyequivalent element capable of timing the CF mode and the QR mode (forexample, based on a counter).

Even if less efficient, the concepts of the present invention are alsoapplicable without the presence of means for switching the switchingelement after a maximum period.

Alternatively, it is possible to use the same circuit for switching theswitching element after the maximum period also at the power-on of theflyback converter (even without a dedicated circuit for this purpose).

Similar considerations apply if the control device has a differentnumber of input and/or output terminals.

It should be readily apparent that the proposed structure might be partof the design of an integrated circuit. Furthermore, the proposedsolution may be integrated with other circuits in the same chip, or itmay be mounted in intermediate products. In any case, the integratedcircuit is suitable to be used in complex systems such as computersystems, control systems, and in portable electronic devices where Vinwould typically be supplied by a battery and the rectifier block 110omitted.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention.

1. A control device for controlling a switching power supply, theswitching power supply being adapted to convert an input voltage into anoutput voltage according to a switching rate of a switching element,wherein the control device includes: first control means for switchingthe switching element in a first working mode at a constant frequency;second control means for switching the switching element in a secondworking mode at a variable frequency, under a maximum frequency, inresponse to the detection of a predefined operative condition of theswitching power supply; and means for selecting the first working modeor the second working mode.
 2. The control device of claim 1, furtherincluding timing means for generating a timing signal having a timingfrequency, the constant frequency and the maximum frequency depending onthe timing frequency.
 3. The control device of claim 1, wherein theswitching power supply has a flyback architecture, and includes atransformer having a primary winding for selectively receiving the inputvoltage through the switching element, and a secondary winding forproviding the output voltage, the second control means including:detection means for detecting the operative condition by detecting azero current condition corresponding to a substantial null currentflowing through the secondary winding.
 4. The control device of claim 3,wherein the switching power supply includes an auxiliary winding coupledwith the primary winding for providing an auxiliary voltagecorresponding to the input voltage, the auxiliary voltage oscillating inresponse to the zero current condition with a succession of stationarypoints, and wherein the detection means includes means for detecting thestationary points in the auxiliary voltage.
 5. The control device ofclaim 4, wherein: the first control means includes means for switchingthe switching element in response to the timing signal; and the secondcontrol means includes means responsive to the timing signal fordetecting a first stationary point in the auxiliary voltage after aminimum period corresponding to the maximum frequency and means forswitching the switching element in response to said first stationarypoint.
 6. The control device of claim 5, wherein: the timing meansinclude an oscillator for generating the timing signal; and the secondcontrol means further includes means for resetting the oscillator inresponse to the switching of the switching element.
 7. The controldevice of claim 5, wherein the second control means further includesmeans responsive to the timing signal for switching the switchingelement after a maximum period depending on the timing frequency withoutthe detection of any stationary point in the auxiliary voltage.
 8. Thecontrol device of claim 1, wherein the second control means furtherincludes means for switching the switching element in response to apower-on of the switching power supply.
 9. The control device of claim2, further including a first input terminal for receiving a frequencyindication, the timing means including means for setting the timingfrequency according to the frequency indication.
 10. The control deviceof claim 4, further including a second input terminal for receiving aworking mode indication, the means for selecting being adapted to selectthe first working mode or the second working mode according to saidworking mode indication.
 11. The control device of claim 10, wherein thesecond input terminal is further adapted to receive the auxiliaryvoltage for the detection means, the control device further includingmeans for discriminating the working mode indication and the auxiliaryvoltage at the second input terminal.
 12. A power conversion system,including: a switching power supply adapted to convert an input voltageinto an output voltage according to a switching rate of a switchingelement; and a control device coupled to the switching power supply, thecontrol device operable in a first constant frequency mode to controlthe switching element at a constant frequency and operable in a secondquasi-resonant mode to control the switching element responsive to aswitching voltage developed on a node of the switching element and astate of a first clock signal generated by the control device andwherein the control device is further operable to select the firstworking mode or the second working mode responsive to a selectionsignal.
 13. The power conversion system of claim 12 wherein the controldevice further comprises an exceptions management circuit operable toactivate the switching element if the switching element has not beenactivated responsive to the switching voltage after a predeterminedtime.
 14. The power conversion system of claim 12, wherein the controldevice includes quasi-resonant circuitry comprising: an internal clockgenerator adapted to receive a set signal and operable to generate aninternal clock signal having a frequency determined by a value of theset signal; an oscillator coupled to the internal clock generator andoperable to generate the first clock signal responsive to the internalclock signal; a quasi-resonant circuit adapted to receive an auxiliaryvoltage have a value that is a function of the value of the switchingvoltage and adapted to receive the first clock signal, thequasi-resonant circuit operable to generate a quasi-resonant drivesignal responsive to the first clock signal and the auxiliary voltage;wherein the control device includes a selection circuit coupled toreceive the selection signal, the selection circuit operable responsiveto the selection signal to output either the quasi-resonant drive signalor the first clock signal to control activation of the switchingelement.
 15. The power conversion system of claim 14 wherein the controldevice is further adapted to receive an open voltage developed on a nodeof the switching element and is operable to deactivate the switchingelement responsive to the open voltage reaching a threshold value. 16.The power conversion system of claim 12 wherein the switching powersupply comprises a flyback converter.
 17. An electronic system,comprising: electronic circuitry; and a power conversion system coupledto the electronic circuitry, the power conversion system comprising: aswitching power supply adapted to convert an input voltage into anoutput voltage according to a switching rate of a switching element; anda control device coupled to the switching power supply, the controldevice operable in a first constant frequency mode to control theswitching element at a constant frequency and operable in a secondquasi-resonant mode to control the switching element responsive to aswitching voltage developed on a node of the switching element and astate of a first clock signal generated by the control device andwherein the control device is further operable to select the firstworking mode or the second working mode responsive to a selectionsignal.
 18. The electronic system of claim 17 wherein the electroniccircuitry comprises computer circuitry.
 19. A method for controlling aswitching power supply, the switching power supply being adapted toconvert an input voltage into an output voltage according to a switchingrate of a switching element, wherein the method includes the steps of:selecting a first working mode or a second working mode; switching theswitching element at a constant frequency in the first working mode; andswitching the switching element at a variable frequency, under a maximumfrequency, in response to the detection of a predefined operativecondition of the switching power supply in the second working mode. 20.The method of claim 19 wherein the predefined operative conditioncomprises an oscillation of a switching voltage developed on a firstnode of the switching element.
 21. The method of claim 20 wherein thedetection of the predefined operative condition further comprisesdetection of an auxiliary voltage having a value that is function of thevalue of the switching voltage.
 22. The method of claim 20 whereindetection of the predefined operative condition further comprises:detecting a zero voltage switching condition of the switching voltage;detecting whether a clock signal has a first or a second state; when theclock signal has the second state, activating the switching element; anddeactivating the switching element responsive to an open voltagedeveloped on a second node of the switching element reaching a thresholdvalue.
 23. The method of claim 22 wherein detection of the predefinedoperative condition further comprises when the clock signal has thefirst state, activating the switching element after a predefined numberof zero voltage switching conditions of the switching voltage have beendetected.
 24. The method of claim 23 wherein, activating the switchingelement comprises applying an active driving signal to the switchingelement; and wherein the method further comprises synchronizing theclock signal and the driving signal responsive to the active drivingsignal.
 25. The method of claim 19 wherein switching the switchingelement at the constant frequency in the first working mode comprises:generating a clock signal; activating the switching element responsiveto rising edges of the clock signal; and deactivating the switchingelement responsive to an open voltage developed on a node of theswitching element reaching a threshold value.